The invention relates to absolute humidity meters.
Known as an absolute humidity meter adapted for use in various types of electronic devices such as copying machines and printers is one based on the heat dissipation coefficient of a thermistor. Such conventional art is disclosed in, e.g., Examined Japanese Patent Publication (Kokoku) Hei-4-50529, a special number "Temperature/Humidity Meter Handbook" of a periodical "Transistor Technology" published in Jan. 1, 1985 (see "How absolute humidity meters are used" in Chapter 2), and the like.
Such an absolute humidity meter utilizes the negative temperature coefficient of resistance of a thermistor, and includes two thermistors. One of the thermistors is arranged within an object to be measured, and the other within a closed dry air environment. The two thermistors are connected in series with each other, and self-heated. A temperature difference between the two thermistors caused by the heat of one of the thermistors being dissipated from the object to be measured is detected as a difference in resistance, so that an absolute humidity signal is obtained from a change in voltage corresponding to the difference in resistance. That is, the heat dissipation coefficient of the one thermistor changes with absolute humidity, and the other thermistor has a fixed heat dissipation coefficient that is in accordance with the dry air, so that a difference in resistance between both thermistors can be detected as the absolute humidity. The other thermistor provides temperature compensation.
However, the aforementioned conventional absolute humidity meter has not addressed the following problems.
Since two thermistors are used, the heat dissipation coefficients are changed by turbulences present in the object to be measured, which in turn impairs measurement accuracy.
Variations in the heat dissipation coefficients of the two thermistors, location temperature differences, and the like impair measurement accuracy.
The heat dissipation coefficients of the two thermistors must be made equal to each other with changing ambient temperatures. However, to select thermistors whose heat dissipation coefficients are equal to each other is difficult. To eliminate the difference between the heat dissipation coefficients, it is necessary to provide a temperature compensation circuit, which in turn complicates the circuit configuration.
Small analog signals must be processed, which makes the circuits necessary for achieving such temperature compensation function not only complicated but also expensive.